The present invention relates generally to plastic pipes, plates and containers, and more specifically, to a process for manufacturing the same.
Pipes and containers made of chemical resisting plastic materials which are defined for use in the chemical industry are usually made in accordance with the so-called winding method by helically winding a mandrel with one or a plurality of layers of a flat band made of plastic material in such a way that the adjacent edges of the flat band or the flat bands which overlap or engage the adjacent edges are welded together to a pipe with a smooth inner surface by pressing the plastic material together in a softened condition, for example, so that the pipe can be removed from the mandrel after cooling.
The winding method may be basically used for making pipes and plates made of polyvinylidene fluoride (PDVF) plastic material. From the economic point of view limits are set for the use of this material, since this material with a density of 1,78.multidot.cm.sup.-3 is very expensive when comparing with other known mass produced plastic materials.
Since the PVDF material has an excellent thermic stability and an extremely remarkable resistancy against the different chemical influences with low temperatures, as well as high temperatures and has an additional high life span resistancy with respect to the known mass produced plastics, there is considerable interest to economically make, in particular large volume pipes and elements for specific uses in the chemical industry from the material PVDF.
It was tried to reinforce a pipe or a plate made from PVDF plastic material with a wall thickness of 4 mm by means of glass fiber reinforced plastic material, thus providing a final product by combining more expensive and less expensive materials. However, such a solution has been shown of not being usable since such structures made of PVDF and GFK plastic material are used in the chemical industry, in particular in temperature ranges of about 343.degree. K. to 413.degree. K. or about 70.degree. C. to 140.degree. C., respectively, and are exposed to partly very high alternating temperatures of 293.degree. K. to about 413.degree. K., that means, from about 20.degree. C. to 140.degree. C. during the operation. It had been shown that the lifespan and the durability of such a material combination are considerably impaired due to the high alternating temperatures. This becomes clear, in particular in that the PVDF plastic material has a mean linear thermic expansion coefficient of 12.multidot.10.sup.-5 K.sup.-1, while this value is at about 3.6 to 2.0.multidot.10.sup.-5 K.sup.-1 with the glass fiber reinforced polyester resin. The expansion and above all the shrinking of the PVDF plastic material may be 3 to 6 times of the corresponding values of the glass fiber reinforced polyester resin, so that the GFK layer is expanding due to the stronger expansion characteristics and shrinking characteristics of the PVDF plastic material and detach from the thermoplastic inner liner made of PVDF plastic material, after a certain time period.
Against the foregoing background, it is an object of the invention to provide a method for making a pipe or a hollow element from thermoplastic plastic materials, for use in the chemical industry, which on the one hand has the required lifespan resistancy with respect to chemical influences and with respect to temperature alternating stresses and on the other hand can be made in a commercially feasable manner.